Pancreatic cancer is the fourth leading cause of cancer deaths in men and women in the United States. The American Cancer Society estimates that in the United States in 2003, there will be 30,700 new cases of pancreatic cancer and 30,000 deaths. Thus, the prognosis for pancreatic cancer is extremely poor.
Cancer of the pancreas generally develops without early symptoms. If a cancer develops in an area of the pancreas near the common bile duct, its blockage may lead to jaundice (yellowing of the skin and eyes due to pigment accumulation). Sometimes this symptom allows the tumor to be diagnosed at a relatively early stage. At present, only biopsy yields a certain diagnosis. Because of the “silent” early course of the disease, the need for biopsy may become obvious only with advanced disease.
Cigarette and cigar smoking increase the risk of pancreatic cancer; incidence rates are more than twice as high for smokers as for non-smokers. Other risk factors include obesity, physical inactivity, chronic pancreatitis, diabetes, and cirrhosis. Pancreatic cancer rates are higher in countries whose populations eat a diet high in fat. Rates are slightly higher in males than in females.
Surgery, radiation therapy, and chemotherapy are the current treatment options. Depending on the stage of the pancreatic cancer, two or even three types of treatment may be used, either simultaneously or sequentially. However, although these treatments can extend survival and/or relieve symptoms in many patients, they seldom produce a cure.
In addition to the poor efficacy of currently available treatments, these current options are also associated with significant side effects. There are two types of surgery for pancreatic cancer, depending on the goal of the treatment. If the tumor is small enough that there is a chance it can all be removed, then the goal can be to completely excise the tumor. However, the cancer may have already metastasized, in which case even complete removal of the tumor will not present a cure. The second type of surgery is palliative surgery, which is used when the cancer is too widespread to be completely removed, and is designed to relieve symptoms or prevent other future problems.
Radiation therapy is treatment with high energy rays such as x-rays to kill pr shrink cancer cells, using either an internal or external source. Radiation, often combined with chemotherapy, can be used for patients whose tumors are too widespread to be removed surgically. Side effects of radiation therapy include mild skin changes that look like sunburn or suntan, upset stomach, loose bowels, and tiredness.
A wide variety of drugs are used for chemotherapy of pancreatic cancer. Side effects depend on the type of drugs administered, the dosage, and the duration of the treatment. Temporary side effects include nausea and vomiting, loss of appetite, hair loss, and mouth sores. Low blood cell counts from treatment can cause an increased risk of infection, bleeding or bruising after minor cuts, and fatigue. In addition, pain can be a real concern for pancreatic cancer patients.
Accordingly, there is a need for improved methods of prevention and treatment of pancreatic cancer.
As a result of advances in molecular genetics and genomics, a number of genes involved in pancreatic cancer have been identified. Therefore, it would be advantageous to develop a treatment to utilize the specific information which can be obtained from genetic and expression analysis, to develop more effective treatment methods than are currently available.
One recent approach to the treatment of cancer is immunotherapy, which is based on the observation that human tumor cells express a variety of tumor-associated antigens (TAAs) that are not expressed or are minimally expressed in normal tissues. These antigens, which include viral tumor antigens, cellular oncogene proteins, and tissue-specific differentiation antigens, can serve as targets for the host immune system and elicit responses that result in tumor destruction. This immune response is mediated primarily by lymphocytes; T cells in general and class I MHC-restricted cytotoxic T lymphocytes in particular play a central role in tumor rejection. Unfortunately, as evidenced by the high incidence of cancer in the population, the immune response to neoplastic cells often fails to eliminate tumors. The goal of active cancer immunotherapy is to augment anti-tumor responses, particularly T cell responses, in order to effect complete tumor destruction.
Most attempts at active immunization against cancer antigens have utilized whole tumor cells or tumor cell fragments as immunogens. However, this approach does not afford reproducibility or control over the precise antigens included in each immunization.
The cloning of genes encoding TAAs has opened new possibilities for the immunotherapy of cancer based on the use of recombinant or synthetic anti-cancer vaccines: Tsang et al., J. Natl. Cancer Inst. 87: 982-90 (1995); Kawakami et al., Proc. Natl. Acad. Sci. USA 91:6458-62 (1994). In recent years, much effort has been expended on gene therapy as a means of combating cancer. The term “gene therapy” has been used to describe a wide variety of methods using recombinant biotechnology techniques to deliver different materials to a cell. Such methods include, for example, the delivery of a gene, antisense RNA, a cytotoxic agent, etc., by a vector to a mammalian cell, preferably a human cell either in vivo or ex vivo. Most of the initial work has focused on the use of retroviral vectors to transform these cells. This focus has resulted from the ability of retroviruses to infect cells and have their genetic material integrated into the host cell with high efficiency. The retroviral vector is typically a modified virus such as Moloney Murine Leukemia Virus (MMLV), which has had its packaging sequences deleted to prevent packaging of the entire retroviral genome.
However, numerous difficulties with retroviruses have been reported. One problem that has developed was initially seen as a key advantage of the virus, namely, the ability of the virus to integrate into the chromosome. However, such integration can be problematic, depending on the chromosomal site of viral insertion. A number of other viruses that were initially believed to be largely episomal in nature such as adenoassociated virus (AAV) have also turned out to have this property. While advantageous in causing long-term expression, it also provides the potential for problems such as undesirable cellular transformation. The stable transformation of a patient's somatic cells makes it difficult to reverse the treatment regimen if undesirable side effects dictate that it should be stopped.
Problems, have also been encountered in infecting certain cells. Retroviruses typically enter cells through cell surface receptors. If such receptors are not present on the cell, or not present in sufficient numbers, then infection may not be possible, or may be inefficient. These viruses are also relatively labile in comparison to other viruses. Outbreaks of wild-type virus from recombinant virus-producing cell lines have also been reported, with the vector itself causing a disease. Moreover, many of these viruses only allow gene expression in dividing cells. Viral vectors based upon lentiviruses such as the Human Immunodeficiency Virus (HIV) do not have these problems, but concerns remain about using such viruses as vectors.
Other viruses have been proposed as vectors, such as herpes virus. In addition, various non-viral vectors such as ligand-DNA-conjugates have been proposed. Nevertheless, these approaches all pose certain problems. For example, a vector must not itself become a potential source for infection to the individual treated. However, as already mentioned, outbreaks of wild-type retroviruses have been reported in some cell lines. Similarly, the use of herpes virus as a vector has been found to result in persistence of the virus. Furthermore, many of these vectors can contain and express only a relatively small amount of genetic material. This is undesirable for numerous situations in which the ability to express multiple products is preferred.
Poxviruses have been used for many years as vectors, particularly with respect to providing a foreign antigen or self-antigen to generate an immune response in a host. The advantages of the poxvirus vectors include: (i) ease of generation and production; (ii) the large size of the genome permitting insertion of multiple genes; (iii) efficient delivery of genes to multiple cell types, including antigen-presenting cells; (iv) high levels of protein expression; (v) optimal presentation of antigens to the immune system; (vi) the ability to elicit cell-mediated immune responses as well as antibody responses; and (vii) the long-term experience gained with using this vector in humans as a smallpox vaccine.
Attention has focused on orthopox such as Wyeth, NYVAC (U.S. Pat. No. 5,364,773) and modified vaccinia Ankara (MVA). MVA was derived from the Ankara vaccinia strain CVA-1 which was used in the 1950s as a smallpox vaccine. In 1958, attenuation experiments were initiated in the laboratory of Dr. Anton Mayr (University of Munich), comprising terminal dilution of CVA in chicken embryo fibroblast (CEF) cells that ultimately resulted in over 500 passages. The resulting MVA is an attenuated, replication-defective virus, which is restricted to replication primarily in avian cells. Comparison of the MVA genome to its parent, CVA, revealed 6 major deletions of genomic DNA (deletion I, II, III, IV, V, and VI), totaling 31,000 basepairs. Meyer et al., J. Gen. Virol. 72:1031-8 (1991). MVA has been administered to numerous animal species, including monkeys, mice, swine, sheep, cattle, horses and elephants with no local or systemic adverse effects. Over 120,000 humans have been safely vaccinated with MVA by intradermal, subcutaneous or intramuscular injections. MVA has also been reported to be a virulent among normal and immunosuppressed animals (Mayr et al., Zentralb. Bakteriol. 167:375-90 (1978). Accordingly, in addition to utility as a smallpox vaccine, the more attenuated strains are particularly attractive poxviruses for use as vectors for immune modulation and gene therapy.
Consequently, poxviruses can be genetically engineered to contain and express foreign DNA with or without impairing the ability of the virus to replicate. Such foreign DNA can encode protein antigens that induce an immune protection in a host inoculated with such recombinant poxvirus. For example, recombinant vaccinia viruses have been engineered to express immunizing antigens of herpes virus, hepatitis B, rabies, influenza, human immunodeficiency virus (HIV), and other viruses (Kieny et al., Nature 312:163-6 (1984); Smith et al., Nature 302: 490-5 (1983); Smith et al., Proc. Natl. Acad. Sci. USA 80:7155-9 (1983); Zagury et al., Nature 326:249-50 (1987); Cooney et al., Lancet 337:567-72 (1991); and Graham et al., J. Infect. Dis. 166:244-52 (1992)). Recombinant vaccinia viruses have also been shown to elicit immune responses against influenza virus, dengue virus, respiratory syncytial virus, and human immunodeficiency virus. Pox viruses have also been used to generate immune reactions against tumor-associated antigens such as EEA, PSA and MUC. See also U.S. Pat. No. 5,656,465.
However, because poxviruses are cytoplasmic viruses and thus DNA does not generally integrate into the host cell's chromosomes, pox vectors are usually not the first choice for gene therapy where a continuous source of an agent is needed for an extended period of time.
In view of the very poor prognosis of pancreatic cancer, and the lack of significant survival provided by the currently available treatments, there is a need for improved methods of treatment for pancreatic cancer.